The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not admitted to describe or constitute prior art to the present invention.
It is often desirable to extend the release time of an injected drug to increase its duration of action or to reduce its toxic effects. Formulations that are readily soluble in the body are usually absorbed rapidly and provide a sudden burst of the pharmacologically active product as opposed to a more desirable and gradual release of the pharmacologically active product. Although a variety of attempts have been made to provide controlled and extended release of pharmacologically active products they have not succeeded in overcoming all of the problems associated the technology, such as achieving an extended release time, maximum stability and efficacy, reduced toxicity, maximum reproducibility in preparation, and the elimination of unwanted physical, biochemical, or toxicological effects introduced by undesirable matrix materials.
Tilmicosin is an antibiotic in the macrolide class with the following structure:

Tilmicosin is effective against a broad range of bacteria, and is used for the treatment of respiratory diseases in cattle. The free form is moderately soluble in aqueous solutions, while the chloride and phosphate salts are highly soluble. At elevated levels, however, tilmicosin is cardiotoxic and its use in sensitive species such as cats, goats, pigs, and horses has been almost entirely avoided due to safety reasons. The commercial product, Micotil® (Eli Lilly & Co., Indianapolis, Ind.), is a solution of the di-phosphate salt and is described in U.S. Pat. No. 5,574,020. This formulation is effective in cattle, but the formulation rapidly releases the antibiotic and, therefor, results in toxicity in many species, including horses, pigs, dogs, and cats.
United States published patent application no. U.S. 2004/0023899 A1 discloses pharmaceutical compositions comprising the salt of a pharmacologically active compound with a lipophilic counterion and a pharmaceutically acceptable water soluble solvent combined together to form an injectable composition. The compositions can be used to extend the release time of the pharmacologically active compound when the composition is administered to an animal.